The physical properties of ionic substances
4 marks · NormalState what holds the ions together in the lattice of an ionic compound such as sodium chloride.
Explain, with reference to the lattice structure, why sodium chloride has a high melting point.
Sodium chloride and magnesium oxide have similar lattice structures. Explain why magnesium oxide has a higher melting point.
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(a) Strong electrostatic forces of attraction between oppositely charged ions.
(b) Because the lattice is held together by strong electrostatic forces between oppositely charged ions, a large amount of energy is required to overcome these forces to melt the crystal.
(c) Because magnesium and oxide ions have higher charges (2+ and 2-), so forces of attraction are stronger than in NaCl.
Also accept: ionic bond instead of electrostatic attraction (a); alternative, concise, but accurate explanations
Common mistakes: Confusing molecules and lattices; Stating melting point is high 'because it is ionic' without reference to forces
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Marking points
B1 (a) · 1 mark · States that ions are held by strong electrostatic forces of attraction (or ionic bonds) between oppositely charged ions.
M1 (b) · 1 mark · Mentions strong electrostatic forces acting between the oppositely charged ions in the lattice.
M2 (b) · 1 mark · Explains that a lot of energy is required to overcome these forces to melt the lattice.
B2 (c) · 1 mark · Links higher melting point in magnesium oxide to stronger attraction due to higher charges on ions (2+ and 2- versus 1+ and 1-).
Worked solution
a) The ions are held together by strong electrostatic forces of attraction between oppositely charged ions.
b) In sodium chloride, the ions are arranged in a giant ionic lattice held together by strong electrostatic forces. A lot of energy is needed to break these forces to melt the solid, which is why sodium chloride has a high melting point.
c) In magnesium oxide, there is a stronger attraction because the ions are 2+ and 2-, rather than 1+ and 1-, which means even more energy is needed to break the lattice apart.